Medical implant materials, in particular orthopedic implant materials, must combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient of the implant is relatively young because it is desirable that the implant function for the complete lifetime of a patient. Because certain metal alloys have the required mechanical strength and biocompatibility, they are ideal candidates for the fabrication of prostheses. These alloys include 316L stainless steel, chrome-cobalt-molybdenum alloys (Co—Cr), titanium alloys and more recently zirconium alloys which have proven to be the most suitable materials for the fabrication of load-bearing and non-load bearing prostheses.
To this end, oxidized zirconium orthopedic implants have been shown to reduce polyethylene wear significantly. The use of diffusion-hardened oxide surfaces such as oxidized zirconium in orthopedic applications was first demonstrated by Davidson in U.S. Pat. No. 5,037,438. Previous attempts have been made to produce oxidized zirconium coatings on zirconium parts for the purpose of increasing their abrasion resistance. One such process is disclosed in U.S. Pat. No. 3,615,885 to Watson which discloses a procedure for developing thick (up to 0.23 mm) oxide layers on Zircaloy 2 and Zircaloy 4. However, this procedure results in significant dimensional changes especially for parts having a thickness below about 5 mm, and the oxide film produced does not exhibit especially high abrasion resistance.
U.S. Pat. No. 2,987,352 to Watson discloses a method of producing a blue-black oxide coating on zirconium alloy parts for the purpose of increasing their abrasion resistance. Both U.S. Pat. No. 2,987,352 and U.S. Pat. No. 3,615,885 produce a zirconium oxide coating on zirconium alloy by means of air oxidation. U.S. Pat. No. 3,615,885 continues the air oxidation long enough to produce a beige coating of greater thickness than the blue-black coating of U.S. Pat. No. 2,987,352. This beige coating does not have the wear resistance of the blue-black coating and is thus not applicable to many components where there are two work faces in close proximity. The beige coating wears down more quickly than the blue-black oxide coating with the resulting formation of oxidized zirconium particles and the loss of the integrity of the oxidized zirconium surface. With the loss of the oxide surface the zirconium metal is then exposed to its environment and can lead to transport of zirconium ions into the adjacent environment.
The blue-black coatings have a thickness which is less than that of the beige coating although the hardness of the blue-black coating is higher than that of the beige coating. This harder blue-black oxide coating lends itself better to surfaces such as prosthetic devices. Although the blue-black coating is more abrasion resistant than the beige coating it is a relatively thin coating. It is therefore desirable to produce new and improved compositions that maintain the desirable properties of the blue-black coatings of the prior art (for example, increased abrasion resistance).
As discussed above, U.S. Pat. No. 5,037,438 to Davidson discloses a method of producing zirconium alloy prostheses with a oxidized zirconium surface. U.S. Pat. No. 2,987,352 to Watson discloses a method of producing zirconium bearings with a oxidized zirconium surface. The oxide coating produced is not always uniform in thickness and the non-uniformity reduces the integrity of the bonding between the zirconium alloy and the oxide layer and the integrity of the bonding within the oxide layer. Both U.S. Pat. No. 2,987,352 and U.S. Pat. No. 5,037,438 are incorporated by reference as though fully set forth herein.
In U.S. Pat. Nos. 6,447,550; 6,585,772 and pending U.S. application Ser. No. 10/942,464, Hunter, et al. describes methods for obtaining an oxidized zirconium coating of uniform thickness. Hunter teaches that such is obtained by applying pre-oxidation treatment techniques and by manipulation of substrate microstructure. The use of uniform thickness oxide layer results in increased resistance to corrosion by the action of the body fluids as well as other benefits and is biocompatible and stable over the lifetime of the recipient. U.S. Pat. Nos. 6,447,550; 6,585,772 and pending U.S. application Ser. No. 10/942,464 are incorporated by reference as though fully set forth herein.
The oxidized zirconium surfaces of Davidson and Hunter (henceforth referred as Davidson-type oxidized zirconium composition), while having relatively thick ceramic oxide or nitride layers, did not exhibit thick diffusion hardened zones below the ceramic oxide or nitride. The diffusion hardened zones of Davidson-type oxidized zirconium compositions had thicknesses of at most 1.5-2 microns and typically less depending upon the conditions used to produce the composition. FIG. 1 shows the nano-hardness profile of Davidson-type oxidized zirconium composition (FIG. 1 is taken from M. Long, L. Reister and G. Hunter, Proc. 24th Annual Meeting of the Society For Biomaterials, Apr. 22-26, 1998, San Diego, Calif., USA). The diffusion zone of the Davidson-type oxidized zirconium is between 1.5 to 2 microns. The oxide is approximately 5 microns, hence the totality of the hardened zone in the Davidson oxide is approximately 7 microns. While the resulting compositions of Davidson and Hunter exhibited high wear resistance in comparison to those compositions available in the prior art, there is still room for improvement.
The significant reduction in wear of polyethylene against oxidized surfaces is attributed to the ceramic nature of the oxide. The oxidized zirconium implant typically has a 5 to 6 micron thick ceramic surface (zirconium oxide) that is formed by a thermally driven diffusion process in air. Beneath the zirconium oxide is a hard, oxygen-rich diffusion layer of approximately 1.5 to 2 microns. The totality of hardened zones (oxide plus diffusion hardened alloy) render the implant resistant to microscopic abrasion (for example, from third bodies such as bone cement, bone chips, metal debris, etc.) and slightly less resistant to macroscopic impact (surgical instrumentation and from dislocation/subluxation contact with metallic acetabular shells). The smaller hardening depth of these implants renders them less than optimal for hard-on-hard applications. In a hard-on-hard application such as in a hip joint, the material articulates against itself or another hardened or non-hardened metal instead of polyethylene. The wear rates in such types of implants could be as high as 1 micron per year. With the totality of the hardened zone (oxide and diffusion zone) having a thickness of less than 7 microns, Davidson-type oxidized zirconium implants, although representing the state-of-the-art when originally introduced and still quite useful, have room for improvement in such applications. Hunter et al (U.S. Pat. No. 6,726,725) teaches such hard-on-hard applications for Davidson-type oxidized zirconium components. Hunter '725 teaches that the oxide thickness can be increased up to 20 microns for such applications. But as will be shown herein, Davidson-type oxide compositions having such thicknesses, although highly wear-resistant, can have significant number of oxide layer defects. Such defects can lead to localized spalling of the oxide. Also, in the Davidson-type composition below the oxide, there is a relatively small diffusion hardened zone. Thus, while the Davidson-type compositions exhibited superior wear resistance compared to many conventional materials, there is always room for improvement.
Currently, there are two primary types of hard-on-hard hip implants that are available commercially, namely metal-on-metal and ceramic-on-ceramic. The current standard material of metal-on-metal implants is high carbon Co—Cr alloy. The major concern with the metal-on-metal implant is the metal ion release from the joint and its unknown effects on the physiology of the human body. The advantage of metal-on-metal implants is that they can be used in larger sizes. The larger size of the implant allows greater range of motion. The metal-on-metal implants have also been shown to be useful for resurfacing type of application where conservation of bone is desired. In such larger joints, the conventional or cross-linked polyethylene is not preferred and metal-on-metal may be the only choice available. The larger size requires polyethylene liner to be thinner. A thinner liner may not be mechanically strong, may creep more or may lead to increased wear and osteolysis and eventually failure of the implant.
The other commonly used hard-on-hard implant material is ceramic-on-ceramic. The current standard material of ceramic-on-ceramic implants is alumina. Metal ion release is typically not a concern for these implants. But due to limited toughness and the brittle nature of ceramics, it is difficult to make these implants in larger sizes. The ceramic components have finite probability of fracture thus leading to a potential joint failure and complications associated with the fracture of a joint.
It has been an object of much of the prior art to reduce the metal ion release and minimize the fracture risk by combining metal and ceramic components. Fisher et al (U.S. Patent Application 2005/0033442) and Khandkar et al. (U.S. Pat. No. 6,881,229) teach using a metal-on-ceramic articulation. Fisher et al teach that the difference in hardness between the metallic component and the ceramic component to be at least 4000 MPa. Khandkar et. al. specifically teach use of silicon nitride ceramic components for articulating against the metallic component. In both instances the objective is to lower the wear of mating couples. But in both instances, the fracture risk of ceramic is still significant. The object of the present invention is to eliminate the risk of fracture along with metal ion release. It is eliminated by using a metallic component with ceramic surface and diffusion hardened zone below the ceramic surface. As mentioned in the details of the invention, diffusion hardened composition of present invention provides a solution to the above described problems pertaining to hard-on-hard bearings made from Davidson-type oxidized zirconium, high carbon CoCr (cobalt-chromium) and alumina. In one aspect of invention, the invented composition is applicable in knee joints and in spinal joints where hard-on-hard articulation is desired.
Unlike the Davidson-type oxidized zirconium, the oxidized zirconium composition disclosed herein is significantly less susceptible to damage caused by dislocation and subluxation. Thus, while the application of diffusion-hardened oxide layers such as Davidson-type oxidized zirconium to orthopedic implants represented a great improvement in the art of implant materials, resulting in substantial improvements in abrasion resistance and service life, the new compositions of the present invention represent improvements over the Davidson-type compositions.
Production of a diffusion hardened zone in zirconium (and its alloys) and titanium (and its alloys) has been disclosed previously. One of the approach suggested by Kemp (U.S. Pat. No. 5,399,207) is to oxidize a zirconium alloy in a temperature range of 426° C. (800° F.) to 871° C. (1600° F.) for two hours or more. The approach of Kemp is to run the process longer so that oxygen diffuses farther into the substrate while the oxidation is taking place. The major disadvantage of this approach is higher temperature and prolonged time is required to form a thicker diffusion zone. The higher temperature and prolonged time can lead to microstructural changes in the substrate and to a defective oxide that comprises substantial amounts of cracks and pores. Kemp teaches the application of its method on a Zircadyne 702 substrate. Following the teachings of Kemp, Zircadyne 702 and medical grade Zr-2.5Nb (ASTM F2384) were oxidized at 800° C. The oxide thickness of Zircadyne-702 samples was 10 to 12 micron whereas that of Zr-2.5Nb was approximately 20 microns (FIGS. 2(a) and 2(b)). The diffusion hardened zone on both samples was approximately 25 microns (FIG. 2(c)). The oxide of both samples showed substantial defects in the form of cracks and pores.
In another approach, Davidson (U.S. Pat. No. 5,372,660) teaches oxidizing Ti alloy that contains Zr. The presence of Zr in Ti leads to formation of an oxide and a thicker diffusion zone. Following the teachings of Davidson an alloy of Ti—Zr—Nb (55% Ti w/w, 35% Zr w/w and 10% Nb w/w) and medical grade Zr-2.5 Nb were oxidized in air. The alloy samples were oxidized at 635° C. for 6 hours. FIG. 3 shows metallographic images showing the oxide and diffusion hardened zone. The oxide of both Ti—Zr—Nb and Zr-2.5Nb is cracked. The oxide of Ti—Zr—Nb appears to separate from the substrate at several locations. FIG. 3 (c) shows micro-hardness of diffusion hardened zone. The Ti—Zr—Nb alloy shows approximately 10 to 15 micron thick diffusion hardened zone. The diffusion hardened zone of Zr-2.5Nb is less than 5 microns. Thus following the teachings of Kemp and Davidson, a significant depth of hardening could be obtained but at the cost of substantial defects in the resulting oxide. Kemp teaches a prolonged treatment at elevated temperatures, whereas Davidson teaches changing the chemistry of the alloy to form a thicker diffusion hardened zone. But in both cases the oxide formed is full of defects. Such type of defects in the oxide can compromise integrity of the oxide and may lead to localized spalling. One of the compositions disclosed herein comprises a thick diffusion zone along with a substantially defect-free oxide. The oxide disclosed herein has additional distinctions over the prior art that will be disclosed further in the details herein. The Davidson-type and Kemp-type oxidized zirconium product is an oxide that is predominantly single phase. The oxide of the present invention comprises a secondary phase that is ceramic or oxygen-rich metal. Embodiments of the diffusion hardened zone of the present invention have a layered structure and a preferred hardness profile.
Another approach to produce a diffusion hardened metallic zone is basically one of forming an oxide on the surface of the article by treatment in an oxygen-rich environment, followed by heat treating the article in an oxygen-deficient environment. One of the approaches provided by Takamura (Trans JIM, vol. 3, 1962, p. 10) has been to oxidize a titanium sample followed by treating it in argon gas (i.e., an oxygen deficient environment with a low partial pressure of oxygen). This apparently allows oxygen to diffuse in the substrate and form a thick diffusion zone. Presence of oxygen in the diffusion zone leads to hardening. Another approach suggested by Dong et al (U.S. Pat. No. 6,833,197) is to use vacuum or an inert gas mixture to achieve an oxygen-deficient environment, thereby achieving the diffusion-hardening after oxidation. The preferred temperature specified by both Takamura and Dong et al for oxidation is 850° C. and that for diffusion hardening (vacuum treatment) is 850° C. Dong et al suggest this methodology for titanium and zirconium and titanium/zirconium alloys. One of the problems with these methods, particularly for zirconium alloys, is that the oxidation and diffusion hardening temperatures are significantly high and can lead to thick and cracked (defective) oxide as well as cracks in the substrates after diffusion hardening. Dong demonstrates its method using titanium alloys; no examples for zirconium/niobium-based or titanium/zirconium/niobium-based alloys have been shown.
Both Takamura and Dong et. al. recommend a preferred temperature of oxidation and inert gas/vacuum treatment of 850° C. Following their teachings, samples of Ti-6Al-4V and medical grade Zr-2.5Nb were oxidized at 850° C. for 0.3 hr in air. FIGS. 4(a) and 4(b) show metallographic images after oxidation. The oxide on the Ti-6Al-4V is less than 1 micron thick. The oxide does not seem to adhere well to the substrate. The oxide on Zr-2.5Nb is approximately 12 microns thick and it is cracked. Following the teachings of Dong, both samples were subjected to vacuum treatment under pressure of 10−4 torr and at 850° C. for 22 hours. FIGS. 4(c) and 4(d) show metallographic images after vacuum treatment. In both samples, oxide has dissolved into the substrate. There are no visible cracks in Ti-6Al-4V sample. The crack is still present on the surface of the Zr-2.5Nb sample. The crack appears to have propagated inside the substrate during the vacuum treatment. These types of cracks on the surface can significantly reduce fatigue strength of the alloy. The new composition and method of the present invention overcomes these deficiencies.
In order to further demonstrate the difference in the behavior between Ti and Zr alloys, samples of Ti-6Al-4V and Zr-2.5Nb were oxidized at a lower temperature (600° C. for 75 minutes). These samples were then treated under vacuum (<10−4 torr) at 685° C. for 10 hours. As will be disclosed further herein, the treatment was carried out in such a way that oxide is partially retained on the Zr-2.5Nb substrate. FIGS. 5 (a) and 5(b) show metallographic images of the oxide formed on Ti-6Al-4V and Zr-2.5Nb samples. The oxide on Ti-6Al-4V is less than 0.1 micron whereas it is approximately 3 micron on Zr sample. No cracks are visible on both samples. After vacuum diffusion hardening, oxide on a Ti-6Al-4V sample is completely dissolved whereas approximately 1 micron oxide is retained on a Zr-2.5Nb sample (FIGS. 5 (c) and 5 (d)). FIG. 5 (e) shows the hardness profile of the diffusion zone. Oxygen diffused almost entirely through the Ti alloy sample and thus produced a negligibly small depth of hardening whereas it did produce a significant depth of hardening in Zr alloy. This example further illustrates the differences in Zr and Ti alloys in the Dong process. It is evident from these examples that the range of temperatures that may work for Zr alloys may not be optimal for Ti alloys and vice versa. Dong also teaches a sigmoid shaped hardness profile of the diffusion hardened metallic zone. The sigmoid shaped diffusion hardened zone profile requires almost complete dissolution of the oxide in the substrate. The inventors of the present invention have found that this is not necessary. The inventors have found that in one aspect of this invention, it is advantageous to retain the oxide on the surface during this process. This is accomplished by careful selection of temperature and time for oxidation and subsequent diffusion hardening. Dong does not teach or suggest retention of the oxide on the surface of the sample at the end of the vacuum treatment and obtaining different types of oxygen concentration or hardness profiles other than a sigmoid profile when the oxide is almost completely dissolved.
In another approach of the prior art, Treco (R. Treco, J. Electrochem. Soc., Vol. 109, p. 208, 1962) used vacuum annealing method to completely dissolve the oxide formed on Zircalloy-2 after corrosion testing. The objective of Treco's work was to eliminate the oxide by vacuum annealing and the resultant diffusion zone by acid pickling. Treco neither discloses advantage of retaining the oxide during diffusion process nor discloses an application where such surfaces could be used. Finally, both Dong and Treco do not disclose use of such a technique to form a ceramic oxide and diffusion hardened zone to make a damage resistant medical implant.
The inventors have found that the damage (i.e., wear) resistance of diffusion hardened medical implant compositions can be improved by increasing the thickness of totality of the hardened zones. The resulting diffusion hardened medical implant compositions are new and not disclosed or suggested in the prior art. The desired totality of hardened zones can be achieved by varying the thicknesses of the ceramic oxide (or nitride, or mixed oxide/nitride) and the underlying diffusion hardened zone(s). Additionally, an increase in the thickness of the diffusion hardened zone imparts additional wear resistance desired in hard-on-hard articulation. A thicker diffusion hardened zone exhibits a layered structure in which the concentration of the diffusion hardening species varies with depth. Careful consideration needs to be applied in selecting the temperature and time of oxidation and diffusion hardening to achieve the desired totality of the hardened zones, while retaining (or enhancing) most of the mechanical, and electrochemical properties of the articles. Furthermore, the proper conditions for the processes of manufacture of such compositions are related to the alloy system under consideration. Such hardened alloys are suitable for articulation against soft polymers (such as ultra high molecular weight polyethylene (UHMWPE), cross-linked polyethylene (XLPE), polyurethane, etc and in hard-on-hard bearing applications against like hardened alloys, against CoCr alloys, ceramics (alumina, silicon nitride, silicon carbide, zirconia, etc), other hard materials such as diamond, diamond-like carbon and ceramic coatings (metal-oxides, metal-nitrides, metal-carbides and diamond), etc.
All of the above-referenced U.S. patents and published U.S. patent applications are incorporated by reference as though fully described herein.